Abstract

The vegetables in Johore are grown on several soil types. Heavy metal contents in Johore soils and vegetables have not been fully studied although it is known that the concentration of heavy metals in soils under vegetable cultivation was affected by excessive use of fertilizers, agrochemical and addition of chicken dung. The quality of these vegetables and the soils from heavy metals perspective is important lately. The knowledge of both the total concentration and chemical speciation is necessary to understand the behavior of heavy metals in soil. This study was conducted to determine heavy metal contents in organic and mineral soils cultivated with vegetables and the relationship between heavy metal concentration and some physico-chemical properties. Heavy metal speciation study is also necessary for evaluating the mobility and bioavailability of heavy metals in the soils.
Soil and vegetable samples were collected from several vegetable plots in Kluang and Kota Tinggi. A total of 177 soil samples were collected comprising of 42 organic soil samples, 81 Rengam series, 27 Telemong series and 27 Batang Merbau series soil. The soil samples were taken from 3 depths (0-20 cm, 20-40 cm and 40-60 cm) using a stainless steel auger. Top soil samples (0-20 cm) from the surrounding area, werealso collected in order to determine the background values of heavy metals in the soil as compared to the vegetable cultivated areas. GPS reading was taken at every sampling point. The crops selected were leafy vegetables (39 samples) and fruit vegetables (17 samples).
Soil samples were analyzed for the pH, organic C, cation exchange capacity (CEC) and texture. Available heavy metals were extracted with EDTA, DTPA and 0.1 N HCl extractants. Total heavy metal contents were determined by the aqua regia-method. The heavy metal concentrations were extracted through dry ashing of the vegetable tissues. For the speciation study, 24 cultivated and 12 uncultivated Rengam series samples were extracted using sequential extraction procedures (exchangeable, carbonate, Fe-Mn oxides, organic and residual fractions) as outline by Abollino et al., (2001) which is a modified method from Tessier (1979). The heavy metal (Cd, Cr, Zn, Pb, Cu and Ni) contents in soil and vegetables were determined using Atomic Absorption Spectrophotometer (FAAS). The geographical distribution maps were produced using GIS software (MapInfo).
From the t-test analysis, the pH, OC and CEC values in the top layers of cultivated organic and mineral soils were significantly higher when compared to background soils. Pearson correlation analysis indicates that the mineral soil pH significantly correlated with Cu (r=0.435**, n=45) and Zn (r=0.450**, n=45) concentrations. Only Cu concentration showed significant correlation with carbon percentages of mineral soils with value of r=0.457**, n=45. Copper, Pb and Zn correlated positively with CEC with values of r=0.176*, r=0.242** and r=0.219* (n=45), respectively. Correlation analysis showed that all heavy metals in organic soil correlated positively with soil pH.
The difference between means of heavy metals of mineral soils showed by t-test indicated that metal concentrations at the top layers of cultivated soils were significantly higher compared to the background soils. Data from organic soils showed that only Cd, Cu, Pb and Zn were significantly higher compared to the background soils. Heavy metals such as Cu, Cr, Pb and Ni in organic and minerals soils were still within the normal range of Malaysian soils. The concentrations of these metals were still lower than the 95th Percentile and Dutch target values except for Cd in the all soil types and Zn (138.11 mgkg-1) in Telemong series. From speciation study, the percentages of mobile Cd (32%) was found to be high in cultivated Rengam series compared to other metals. However, the percentages of mobile fractions for all metals in the cultivated soils were lower compared to the uncultivated soils. The majority of metals present were in the residual form (immobile fraction).
Among the vegetables, Indian Pennyworth exhibited the highest mean concentrations of Cd (0.02 mgkg-1), Cr (0.8 mgkg-1), Pb (0.29 mgkg-1) and Zn (10.16 mgkg-1) compared to the other vegetables species. Fruit and leafy vegetable tissues contain highest Zn and the least Cd concentrations. Results of the transfer coefficients showed that leafy vegetables tend to accumulate higher concentration of metals in edible tissue compared to the fruit vegetables. Pearson correlation analysis indicated that total Ni in vegetables grown on Rengam series correlated positively with available Ni in soil using EDTA (r=0.408*, n=27), while Cd in vegetables correlated significantly with available Cd using 0.1N HCl extraction (r=0.615**, n=27). Positive correlation was only obtained between Cr content in vegetables grown on organic soil with available Cr (r=0.566*, n=14) using 0.1N HCl. Results showed that the heavy metal concentrations in fruit and leafy vegetables were still below the maximum permissible concentration (MPC) as stated in the Malaysian Food Act (1983) and Food Regulations (1985).
Distribution maps of heavy metals in Kluang and Kota Tinggi showed that Cd concentrations in vegetable areas were higher than the 95th percentile, which is proposed by Zarcinas et al., (2004) to be Malaysian soil investigative level. One of the vegetable areas in Kluang (C5) showed high concentration of Zn (157 mgkg-1), which is higher than the 95th percentile. This farm had been actively producing leafy vegetable for almost 20 years. It is evident that the vegetable areas in Kluang and Kota Tinggi showed higher content of metal concentration compared to the surrounding areas. The pollution index (PI) values in the vegetable areas in Kluang and Kota Tinggi are still below 1.0, indicating that metal concentrations are below tolerable level. However, these vegetable growing areas must be continuously monitored due to some indication of possible heavy metal contamination.